[noise] selfie attack

[Justin Cormack]

This paper https://eprint.iacr.org/2019/347.pdf points out that (in
Noise terms) NNpsk handshakes and traffic can be reflected back to the
originator if it acts as client and server and for some protocols this
can be a problem. Not sure it is really an issue for the use cases
that might use that, as assuming a psk is an identity is somewhat of a
bad idea.

Justin